So your Wizard can keep all of his metamagic rods at home in his tower hanging up next to his pots and pans. He uses them only at spell-prep time, and not otherwise. He never runs into the problem, especially if he's an NPC, of having his precious rods taken from him alive, or looted from his corpse before his clone can reanimate and fetch his stuff. At least, not when he's out adventuring. The PCs won't be rewarded with looting a metamagic rod when they defeat a wizard who enhanced his spells with one, until they loot his warded keep.
As you like, I guess...
Frankly I prefer the idea that rods enhance the spell the same way that some planes affect spellcasting... that they are changed upon casting, and not upon memorizing. The rods affect the spell like an angreal out of Wheel of Time. And you have to carry it with you all the time to have access to its abilities, and ware the threat of having it taken from you. But that's just me.
As you like, I guess...
Frankly I prefer the idea that rods enhance the spell the same way that some planes affect spellcasting... that they are changed upon casting, and not upon memorizing. The rods affect the spell like an angreal out of Wheel of Time. And you have to carry it with you all the time to have access to its abilities, and ware the threat of having it taken from you. But that's just me.